I don't think you're clear as to what retcon means. If your definition holds, then everytime a show or movie reveals something new about a character, it's a retcon. No, that's just a reveal. A retcon has to contradict something that's explicitly stated and/or shown to be true.
Just because the final 5 talk about their pasts doesn't make those pasts true. And whether or not the writers intended from day 1 for them to be such doesn't make it a retcon either. If we actually SEE a young Saul fighting off cylons and other people say, "yeah, I was there with him, he was badass" and then in season 4 it suddenly didn't happen... THEN it's a retcon.
None of those things are contradicted by the explanation that they were placed in colonial society and given false memories, nothing is shown that proves their pre-reveal histories to be "true" in the same way we see Bill Adama's life as a fighter pilot, as a child, etc. Other characters share those experiences. Nothing in the show, ever, shows saul before he and adama are a bunch of washups which is where their personal history begins. THAT is why it's not a retcon, it doesn't contradict or change anything.
and I never said there weren't "any" retcons, just that the cylons and their count are not retcons. Actual retcons would be like the base star redesigns in the middle of the show.